Recently, a nonaqueous electrolyte battery such as a lithium-ion secondary battery has been actively developed as a battery having a high energy density. The nonaqueous electrolyte battery is expected to be used as a power source for vehicles such as hybrid vehicles or electric cars or an uninterruptible power supply for a mobile phone base station. Therefore, the nonaqueous electrolyte battery is desired to have other performance such as rapid charge-and-discharge performance and long-term reliability. For example, a nonaqueous electrolyte battery enabling rapid charge and discharge not only remarkably shortens the charging time but also makes it possible to improve performance related to motivity and to efficiently recover regenerative energy from motivity, in a hybrid vehicle and the like.
In order to enable rapid charge and discharge, it is necessary for electrons and lithium ions to be able to migrate rapidly between the positive electrode and the negative electrode. When a battery using a carbon-based material in the negative electrode undergoes repeated rapid charge and/or discharge, dendrite precipitation of metal lithium can occur on the electrode. Dendrites cause internal short circuits, which can lead to heat generation and fires.
In light of this, a battery using a metal composite oxide in the negative electrode in place of a carbonaceous material has been developed. Particularly, in a battery using titanium oxide as the negative electrode active material, rapid charge and discharge can be performed stably. Such a battery also has a longer life than those using a carbonaceous material.
However, titanium oxide has a higher (nobler) potential relative to metal lithium than that of the carbonaceous material. In addition, titanium oxide has a lower capacity per weight. Therefore, a battery using titanium oxide in the negative electrode has a problem in that the battery has low energy density.
For example, an electrode potential of an electrode using titanium oxide is about 1.5 V based on metal lithium. This potential is higher (nobler) than that of the electrode using carbon-based negative electrode. The potential of titanium oxide is due to the redox reaction between Ti3+ and Ti4+ when lithium is electrochemically absorbed and released. Therefore, the potential of titanium oxide is limited electrochemically. Further, there is the fact that rapid absorption and release of lithium ions can be stably performed due to an electrode potential as high as about 1.5 V. Therefore, it is substantially difficult to lower the potential of the electrode to improve energy density.
Further, for the capacity of the battery per unit weight, the theoretical capacity of titanium dioxide having anatase structure is about 165 mAh/g, and the theoretical capacity of a lithium-titanium composite oxide such as Li4Ti5O12 is about 180 mAh/g. On the other hand, the theoretical capacity of a general graphite-based electrode material is not less than 385 mAh/g. Therefore, the capacity density of titanium oxide is significantly lower than that of the carbon-based negative electrode. This is due to a reduction in substantial capacity because there are only a small number of lithium-absorption sites in the crystal structure and lithium tends to be stabilized in the structure.